+1 for mentioning straining the liquid. I've forgotten to do that, and you end up with a very crunchy sauce.
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TaerareshOct 28 '10 at 1:44

2

For some mushrooms the necessary "a while" can be quite long, e.g. 30 minutes. Be careful with the liquid, depending on the mushroom its taste may not be what you are looking for and you may want to discard it.
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user2215Nov 1 '10 at 0:15

Do you have to soak them in a deep pan? So that the grit remains on the bottom? I too recently used some dried porcinis and my pan sauce ended up very gritty. The package said to 'blanche' them.
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MVCylonJun 19 '13 at 12:41

I agree with all the answers above, but I must make some additional steps to it. I also believe we are talking whole mushroom. If it is sliced mushrooms, you will need to soak them for less time.

Yes, it's best to use cold water and it will take at least couple of hours.

I would soak them in cold water for about 15 mins

Pour all the water out. Put in fresh water and keep soaking it. I will do this step couple of times. Remember you may save the water to use as flavoring, but you have to clean the mushroom and make sure the bitterness is gone. Also, I must say the imported Chinese mushroom must be handled carefully as there have been reports that they may contain dangerous chemicals.

After a couple of hours, make sure the mushrooms are soft, take them out of water, and drive them thoroughly. You may keep the water if you wish.
The mushrooms are good for foods that are steamed or stir fried.

Hot water

If you are in rush, then you may use hot or warm water. You still have to go through the cleaning process, but the time will be less.

Soup

If you use mushrooms for making soup, you may not need to soak it. I would soak them for quick 10 mins and give them a clean. Throw them straight into water and boil them to make soup. The favor of the mushrooms will come out nicely.

Perhaps not the 'best' way to use dried mushrooms, but if you powder them up in your spice grinder, they make an excellent thickening agent for soups/sauces. You won't get thickening at the level of cornstarch or arrowroot, but it will thicken things and impart a mushroomy goodness.

Depending on the mushrooms, it may take a little longer. You could soak them in warm/hot water and it'll be quicker, but if I have the time I generally soak them in cold water for about an hour. I'm told it retains much more of the flavour when you do that.